Association between Free Thyroxine Levels and Diabetic Retinopathy in Euthyroid Patients with Type 2 Diabetes Mellitus

Background: This study aimed to evaluate the association between thyroid parameters and diabetic retinopathy (DR) in euthyroid patients with type 2 diabetes mellitus (T2DM).Materials and Methods: In this cross-sectional study, a total of 911 euthyroid patients with T2DM (539 men and 372 women; mean age, 60.81 ± 12.93 years) were enrolled. Clinical factors were assessed and free triiodothyronine (FT3), free thyroxine (FT4) and thyroid-stimulating hormone (TSH) levels were measured. DR was diagnosed using fundus fluorescein angiography.Results: Compared with patients without DR (n = 718), patients with DR (n = 193) exhibited lower FT3 (4.40 ± 0.58 vs. 4.50 ± 0.51 pmol/L; P = .019) and FT4 (14.86 ± 2.09 vs. 15.91 ± 2.18 pmol/L; P < .001) and higher TSH (1.86 [1.22, 2.66] vs. 1.58 [1.14, 2.34] µIU/mL; P = .015) levels. After adjustment for potential DR risk factors, patients in the highest tertile of plasma FT4 levels had a 0.332-fold likelihood of developing DR compared with those in the lowest tertile of plasma FT4 levels (Ptrend < 0.001). The prevalence of DR showed a significantly decreasing trend across the three tertiles based on FT4 levels (31.35%, 19.08% and 13.16%; Ptrend < 0.001). Similar results were obtained for the presence of proliferative DR.Conclusion: These findings suggest that low-normal FT4 levels are associated with the prevalence of DR in euthyroid patients with T2DM.